The invention concerns a vehicle tire and, notably, a tire whose architecture is optimized in order to reinforce its endurance and speed resistance.
It is now common for tires of passenger cars designed to run at high speeds to embody an additional ply of circumferentially oriented cords. That ply can be placed above the crown reinforcement plies at angles also commonly used.
In such a configuration, that ply of circumferentially oriented cords is the radially outermost ply of the tire crown.
A break of one or more of those circumferential cords on rolling can shorten the lifetime of the tire. In fact, such breaks reduce the binding of the crown, but can also lead to seepage of water along the circumferential cords, which can entail wire corrosion phenomena.
A zone sensitive to circumferential cord ply damage is the lateral end of the working ply closest to the circumferential cords, owing to the short distance between those circumferential cords and the ends of the wire cords of the working ply. The damage to the circumferential cords can be due, notably, to abrasive contacts with the ends of the cords of the ply at the outermost angle.
Tire designers have proposed various arrangements for keeping the circumferential cords away from the ends of the reinforcing plies at angles from the crown, notably, through the addition of rubber flanging layers.
These arrangements make the tire more difficult to manufacture and increase its cost. Furthermore, the shaping operations make industrial control of these complementary operations subject to time variations.
The object of the present invention is to present an economical arrangement for solving that technical question.
In what follows, xe2x80x9ccordxe2x80x9d is understood to mean monofilaments as well as multifilaments, or assemblages like cables, yarns or even any type of equivalent assemblage, whatever the material and treatment of those cords, like, for example, surface treatment or coating or pre-sizing to promote adherence to the rubber.
xe2x80x9cRubber bonding layerxe2x80x9d for a given reinforcing ply is understood to mean the rubbery compound in contact with the ply reinforcing cords, adhering to the latter and filling the interstices between adjacent cords.
xe2x80x9cContactxe2x80x9d between a cord and a rubber bonding layer is understood to mean the fact that at least one part of the outer circumference of the cord is in close contact with the rubber compound constituting the rubber bonding.
xe2x80x9cTiterxe2x80x9d is understood to mean the weight in grams of one thousand meters of a cord. The titer is expressed in tex. The stress undergone by a cord or the modulus of that cord is expressed in xe2x80x9ccN/texxe2x80x9d, cN meaning centinewton.
xe2x80x9cLaying pitch pxe2x80x9d of a generally circumferentially oriented and spiral-wound cord is understood to mean the distance transversely separating the axes of two adjacent cord turns. The laying pitch is the inverse of xe2x80x9claying density d,xe2x80x9d which corresponds to the number of cord turns contained in a given axial width: p=1/d. Usually, d is expressed in number of cords per decimeter (f/dm) and p by millimeters; thus, p=100/d.
The xe2x80x9chigh-temperature contraction potentialxe2x80x9d of a textile cord called xe2x80x9cCSxe2x80x9d is understood to mean the relative variation of length of a textile cord positioned, under a prestress equal to the half-sum of the titers of each of the elementary fibers, between the platforms of a furnace (TESTRITE type apparatus) regulated at a constant temperature of 185 xc2x10.5xc2x0 C. The CS is expressed in % by the following formula:
CS(%)=100xc3x97|L1xe2x88x92L0|/L0 
where L0 is the initial length of the adhered cord at room temperature under a prestress equal to the half-sum of the titers of each of the elementary fibers and L1 is the length of that same cord at 185xc2x0 C. The length L1 is measured at the end of a cord stabilization time at temperature of 185xc2x0 C., equal to 120 sxc2x12%. The standard deviation on the measurement of CS is xc2x10.15%.
That potential is a direct consequence of the set of operations that the cord has undergone on its elaboration or on its use.
The invention concerns a tire containing a crown extended by two sidewalls and two beads, a carcass anchored in the two beads, in which the crown radially comprises from inside:
at least one reinforcing ply of axial width L, formed by parallel cords oriented at an angle xcex1 relative to the circumferential direction of between 10 and 75 degrees, and
at least one ply of cords obtained by spiral winding of the cords in a generally circumferential direction, arranged radially outside relative to the reinforcing ply and extending axially beyond the reinforcing ply.
This tire is characterized in that, in the zones axially placed relative to the equatorial plane P of the tire at a distance ranging between L/2xe2x88x92h and L/2+H, the laying pitch of the cords is greater than or equal to the axial distance H+h, in that the values of H and h are equal or over 2 mm and in that said ply of cords arranged in a generally circumferential direction has an axial width strictly over L+2 H.
The fact that in this zone the laying pitch of the circumferentially oriented and spiral-wound cords is greater than or equal to the axial distance H+h means that, from one side of that zone to the other, only one circumferential cord turn has been arranged at most. This embodiment therefore appreciably diminishes the likelihood of an accidental break of that cord due to an abrasive contact between that cord and a cord of the adjacent ply and thus reinforces the resistance of the tire as a whole.
The invention also concerns a similar tire in which the ply of circumferential cords comprises, at least on one side of the equatorial plane P of the tire, a first spiral winding extending from the equatorial plane P to an axial distance of L/2xe2x88x92h and at least a second spiral winding axially extending outward beyond an axial distance L/2+H from the equatorial plane of the tire.
That embodiment has the advantage of not entailing any risk of contact between the cords of the circumferential ply and the axial end of the cords of the reinforcing ply formed by cords oriented at angle xcex1. Industrial production is, however, somewhat longer by reason of the interruption and resumption of laying of the circumferential cords.
The laying pitch of the ply of generally circumferentially oriented spiral-wound cords is advantageously less in the zone axially placed outside beyond the axial distance L/2+H than the laying pitch of the cords in the center zone of the tire crown.
That makes it possible to compensate for the absence of binding opposite the axial end of the crown reinforcing ply of width L in order to obtain a uniform binding. Binding can also be increased in the shoulder zone in order to improve resistance to high speeds of the tire.
For a similar purpose, the laying pitch of the circumferential cords can also be reduced in the zone placed axially placed in proximity to the axial distance L/2xe2x88x92h.
When the tire crown includes a second reinforcing ply of axial width Lxe2x80x2, radially inward from the first, formed by parallel cords oriented at an angle xcex2 of between 10 and 75 degrees, the ply of circumferential cords can extend axially within or beyond distance Lxe2x80x2/2 from the equatorial plane of the tire, depending on the level of high-speed resistance sought.
According to one particular embodiment, the tire tread is, on at least a given axial zone of the crown, in direct contact with the circumferentially oriented cords. That facilitates manufacture of the tire by reducing the number of products to be laid. When the tread contains a first compound intended to come in contact with the road, as well as an underlayer radially placed under the first compound, it is the underlayer which is advantageously in direct contact with the circumferentially oriented cords.
The so-called underlayer compounds are intended to improve different performances like the consumption or drift rigidity of the tires.
The ply of generally circumferentially oriented cords advantageously consists of at least two simultaneously spiral-wound cords. That makes it possible to reduce the ply laying time. The maximum number of cords that can be simultaneously wound is at most four.
The simultaneous winding of two to four cords makes it possible to reduce the laying time of the ply of circumferential cords appreciably, since the laying pitch is doubled, while maintaining the same cord density. That does not appreciably increase the likelihood of abrasive contacts between the circumferentially oriented cords and the end of the cords oriented at xcex1 in the critical zone. Beyond four cords laid simultaneously, the length of contact possible increases, so that the benefit of the invention disappears.
Preferably, h ranges between 2 and 10 mm, and H is greater than 2 mm.
The generally circumferentially oriented and spiral-wound cords advantageously develop a stress under 3% strain greater than 12 cN/tex and preferably greater than 20 cN/tex. Those cords therefore present a high modulus of elasticity at elevated strains, which makes it possible for the ply they form to assume all its functions, notably, binding of the crown at high speed, while maintaining a relatively low laying density.
Those cords can present an initial modulus less than 900 cN/tex and preferably less than 800 cN/tex. The low initial modulus of those cords has the advantage of improving comfort of the tire and reducing its low-speed coast by running noise.
Such a cord can be a hybrid cable combining at least one nylon yarn and at least one aramid yarn.
The circumferentially oriented cords advantageously present a standard contraction potential at high temperature below 3.5%. That makes it possible to limit their contraction on vulcanization of the tire and thus reinforce in all the working variants described the likelihood of avoiding, in operation, all abrasive contact between the circumferentially oriented cords and those oriented at a and radially arranged inward.
It is also advantageous to spiral-wind the cords with laying diameters roughly corresponding to the final diameters of the cords in the tire after vulcanization. As previously, that reinforces the probability of avoiding, in operation, any abrasive contact between the circumferentially oriented cords and those oriented at a and radially arranged inward.
The embodiments described also have the advantage of making it possible to reduce the quantity and thickness of the rubber flanging layers. This is favorable in terms of thickness and heating at the shoulder.